latter of which is thought to proceed via an unusual al ##ky ##ne @ - @ allen ##e iso ##mer ##ization that generates the di ##vin ##yl ke ##tone . = = = short ##coming ##s = = = the classical version of the na ##zar ##ov cy ##cl ##ization suffers from several draw ##backs which modern variants attempt to ci ##rc ##um ##vent . the first two are not evident from the mechanism alone , but are indicative of the barriers to cy ##cl ##ization ; the last three stem from select ##ivity issues relating to elimination and proton ##ation of the intermediate . strong lewis or pro ##tic acids are typically required for the reaction ( e . g . ti ##cl ##4 , bf ##3
latter of which is thought to proceed via an unusual alkyne @ - @ allene isomerization that generates the divinyl ketone. = = = shortcomings = = = the classical version of the nazarov cyclization suffers from several drawbacks which modern variants attempt to circumvent. the first two are not evident from the mechanism alone, but are indicative of the barriers to cyclization ; the last three stem from selectivity issues relating to elimination and protonation of the intermediate. strong lewis or protic acids are typically required for the reaction ( e. g. ticl4, bf3